Low expansion zinc petalite-beta quartz glass-ceramic articles

ABSTRACT

THIS INVENTION RELATES TO THE MANUFACTURE OF GLASSCERAMIC ARTICLES EXHIBITING VERY LOW COEFFICIENTS OF THERMAL EXPANSION WHICH HAVE COMPOSITIONS IN THE ZN0-AL2O3SIO2 FIELD. THE ESSENTIAL ABSENCE OF ALKALI METAL IN THE COMPOSITIONS ENDOWS THE PRODUCTS OF THIS INVENTION WITH HIGH ELECTRICAL RESISTIVITY.

United States Patent 3, 1, 1 LOW EXPANSION ZINC PETALITE-BETA QUARTZ GLASS-CERAMIC ARTICLES George H. Beall, Coming, and Francis W. Martin, Painted losgNliY assignors to Corning Glass Works, Commg No Drawing. Filed Aug. 19, 1970, Ser. No. 65,264

Int. Cl. C04b 33/00 U.S. Cl. 106-39 DV Claims ABSTRACT OF THE DISCLOSURE This invention relates to the manufacture of glassceramic articles exhibiting very low coefficients of thermal expansion which have compositions in the ZnO-Al O SiO field. The essential absence of alkali metal in the compositions endows the products of this invention with high electrical resistivity.

The production of glass-ceramic articles involves the carefully controlled crystallization of glass articles through heat treatment thereof. In general, the manufacture of glass-ceramic articles contemplates three steps. First, a glass-forming batch of a desired composition, to which a nucleating agent is commonly added, is melted. Second, the melt is simultaneously cooled to a glass and an article of a desired configuration shaped therefrom. Third, the glass article is exposed to a predetermined heat treating schedule to cause the crystallization in situ of relatively uniformly-sized, fine-grained crystals homogeneously dispersed in a residual glassy matrix. Normally, this heat treatment comprises two steps. First, the glass article is exposed to a temperature above the softening point of the glass to cause the growth of crystals on the nuclei.

lnasmuch as a glass-ceramic article is formed through the crystallization in situ of a glass article, it is free from voids and non-porous. Further, since a glass-ceramic article is commonly predominantly crystalline, i.e., greater than 50% by weight crystalline, the chemical and physical properties thereof are more closely akin to those of the crystal phase than those of the original glass. Finally, the residual glassy matrix has a composition quite dissimilar from that of the parent glass article because the components thereof constituting the crystals have been precipitated therefrom.

The instant invention is founded upon the discovery that certain glasses in the ZnO-AI Og-SiO composition field, when nucleated with ZrO and/or the noble metals, can be crystallized in situ to yield glass-ceramic articles wherein zinc petalite solid solution and/or beta-quartz solid solution comprise the predominant crystal phases. Zinc petalite has been so called because the pattern thereof observed in X-ray difi'raction analysis very closely approximates that of petalite (LiAlSi O It is deemed to be a solid solution phase composed of a combination of the following species: ZnAl -Si O ZnSi 0 and ZnAl- Si O which are zinc analogs of petalite.

In its broadest terms, the invention contemplates melting a batch for a glass consisting essentially, by weight on the oxide basis, of about 13-40% ZnO, 926% A1 0 40-75% SiO and 310% ZrO and/or O.001-O.5% of a noble metal selected from the group consisting of copper, gold, platinum, palladium, and silver. The melt is simultaneously cooled to at least below the transformation range thereof and a glass article shaped therefrom and, thereafter, this glass article is exposed to a temperature within the range of about 775 95 0 C. for a period of time suflicient in length to secure the desired crystallization in situ. The transformation range is that temperature at which a liquid melt is deemed to have become 3,681,097 uPatented Aug. 1, 1972 tween the strain: point and annealing point of the glass.

vInasmuch as this crystallization is a time-temperature dependent process, at temperatures within the upper extreme of the crystallization range, short exposure periods only will be required, e.g., about one hour or even less. However, at temperatures within the cooler extreme of the crystallization range, exposure times as extended as 24-48 hours may be necessary to achieve extensive crystallization.

Where heat treating temperatures much above about 950 C. are employed, the zinc petalite and beta-quartz solid solutions will break down into such crystal phases as gahnite (ZnO-Al Os), Willemite (ZZnO 'SiOZ), and cristobalite (SiO The development of these phases destroys the very low thermal expansion character of the products. Hence, whereas these crystals may be present in the final product even when the lower heat treating temperatures are employed, the quantities thereof are so small as to have little effect upon the overall properties of the crystallized product.

The preferred heat treatment practice involves first exposing the glass article to a temperature somewhat above the transformation range, e.g., 725-800 C., and maintaining thereat for a period of time of suflicient length to assure extensive nucleation and initiate crystal growth. Subsequently, the article is heated to about 800-9=50 C. and held within that temperature range for a sufiicient length of time to promote essentially complete crystal growth. In general, a nucleation time of 2.-6 hours followed by a crystallization growth period of about 1-8 hours has been found to yield uniformly fine-grained, highly crystalline products.

It can be appreciated that various modifications in the manufacturing procedure are possible. For example, after the melt is cooled below the transformation range thereof and shaped to a glass article, the article may then be further cooled to room temperature to permit visual inspection of glass quality prior to beginning the heat treating operation. Nevertheless, where speed of production and fuel economies are of paramount importance, the melt may merely be quenched to a glass shape at a temperature immediately below the transformation range and the crystallization practice initiated at once.

Also, whereas a two-step heat treatment schedule is to be preferred, a satisfactorily-crystallized article can be produced when the glass shape is simply heated from room temperature or the transformation range to temperatures of 775950 C. and maintained Within that range for a sutficient length of time to develop a highly crystalline product. Commonly, articles so-produced will not be as uniformly fine-grained as will be secured in the two-step procedure.

Finally, when the rate of heating the glass article from room temperature or the transformation range is not too rapid and the final crystallization temperature is near the upper extreme of the crystallization range, no dwell period at any one particular temperature will be required. However, inasmuch as the growth of crystals is time and temperature dependent, the rate of heating the glass article above the transformation range must not exceed the rate of crystal growth so that an insuflicient number of crystals are developed to support the article. The lack of crystallization will result in deformation and slumping of the glass article when the softening point thereof is approached and exceeded. Hence, whereas heating rates of 10 C./ minute and higher have been utilized successfully, particularly in instances where physical supports such as formers have been provided to inhibit deformation of the glass articles, heating rates of about 3-5 C./minute are to be preferred. These latter heating rates have yielded articles demonstrating little, if any, physical deformation product. Whereas their total absence is desirable, up to throughout the composition field composing the invention. about 2% by weight can be tolerated as a melting aid.

Table I reports compositions, expresed in weight per- Additions of the alkaline earth metal oxides MgO, CaO, cent on the oxide basis, of thermally crystallizable glasses and SrO should also be avoided due to their solid solution which were heat treated in accordance with the method 5 in the zinc petalite and beta-quartz phases or the developof this invention. The actual batch ingredients can comment of unwanted new phases such asspinel prise any materials, either oxides or other compounds, which, on being melted together, are converted to the (MgO-Al O desired oxide compositions in the proper proportions.

Since the amounts of the noble metals are so minor, they No more than about 2% of any one individually or more are merely reported as being in excess of the base glass than 3% total of the group can be tolerated. Finally, composition. The batch ingredients were compounded, very minor amounts of TiO can be substituted for ZrQ ballmilled together to aid in securing a homogeneous melt, as the nucleating agent but such substitution tends to favor and then melted in open platinum crucibles for about 16 the developement of the undesirable gahnite. TiO is a hours at temperatures between about l500-1600 C. more efiicient nucleating agent than ZrO Therefore, here Glass cane having a diameter of about /4" were hand again, no more than about 2% by weight can be tolerated drawn from each melt and the remainder poured onto a and, preferably, at least 5% ZrO should'also be present steel plate to produce a circular glass patty about 5" in to insure the extensive growth of zinc petalite and betadiameter and /2" in thickness. The glass patties were quartz.

transferred immediately to an annealer operating at 650 20 At least 3% Zr0 or 0.001% noble metal must be in- C. The annealed patties and cane were thereafter placed cluded in the glass composition to initiate sufiicient nucleain an electrically-fired furnace and subjected to the firing tion therein to attain a uniformly fine-grained crystalline schedules reported in Table II. At the conclusion of each body. More than 10% ZrO hazards batch melting probheat treatment, the electric current to the furnace was cut lems and is unnecessary since the tfinal product simply off and the crystallized articles either removed directly contains more crystallization of tetragonal zirconia.

from the furnace into the ambient atmosphere or merely Amounts of noble metal in excess of 0.5% are useful and left within the furnace and allowed to cool to room temdo not harm the crystallized product but are unnecessary perature therein. This latter practice, termed cooling at and economically unattractive. SnO in amounts up to about furnace rate, was estimated at an average rate of cooling 2% are frequently useful in lowering the liquids and reof about 35 C./minute. ducing the noble metal.

Although the above-recited amounts of ZnO, A1 0 The melts of the glasses recorded in Table I were quite SiO and nucleating agent are demanded to obtain a highfluid so no fining agent, as such, was required. Neverthely crystalline glass-ceramic article of essentially zero exless, in large scale commercial melting, a conventional finpansion with zinc petalite solid solution and/or beta-quartz ing agent such as As O can be added as needed. In acas the predominant crystal phases, minor amounts of comcordance with conventional glass analytical practice, the patible metal oxides totalling not more than about 10% noble metals are reported in metallic form.

TABLE I ZnO,percent-. 23 82.56 17. 02 18.69 16.01 20.66 A1203, percent- 2 9.30 17.92 16.36 16.01 18.78 e102, percent 4 51.16 53.79 58.37 61.80 53.50 ZlOz, percent.-.-- 6.98 5.39 3.74 4.74 5.63 ASzOa,pelG6I1t 0. so 0 0. 50 0.50 0.50 0.50 SnO, percent. Pd, percent.- Pt, percent. Au, percent. Ag, percent 0.00. BaO, percent.-. 4.48 2. 34 BeO,percent 0.94 0.93

by weight may be included to aid in melting the batch or 0 Table II tabulates the particular heat treatment schedule to modify the chemical and/or physical properties of the utilized in crystallizing each glass article to a white, finecrystallized body. Hence, additions of up to 6% by weight grained opaque or translucent, crystalline article and sev- BaO are useful in stabilizing the coefiicient of expansion eral measurements of coeflicients of thermal expansion on of the products. BeO in amounts up to about 3% can be the range 25 -600 C., dielectric constants and loss tanadded to lower the liquidus of the melt and improve the 5 gents determined at 25 C. and 1 kilocycle, electrical resurface quality of the final products. Also, B 0 P 0 sistivities measured at 400 (3., and identifications of the and PhD are effective as fluxes but to assure good thermal crystal phases present as determined through X-ray difstability should not exceed a total of about 5% by weight. fraction analyses. In each reported schedule, the tempera- The presence of the alkali metal oxides, e.g., Li O, Na O, ture was raised at a rate of about 5 C./minute to the and K 0, should be avoided due to their extremely deledwell temperature and the crystallized articles cooled to terious elfect upon the dielectric properties of the final room temperature at furnace rate.

TABLE II Example Exp. coefi. Dielectric Loss No. Heat treatment Crystal phases (XIO-U" G.) constant tangent Log R 1 ..{2 Egg: ggg: 8:::}B eta-quartz 3. 0 6.63 0.0017 10, 8 2 {Z Egg: 2% 3%: 8-:-"}Beta-quartz, zinc petall e 1.0

. 21mins at 800 0: a hours }zmc petame 6. 0

2 hours at 775 0.... 4 2 hours at 825 0.... Zinc petallte .1 9.0

4 hours at 875 0..--

2 hours at 825 0..-. 4 hours at 875 0--.. 2 hours at 775 0...;

6.4:: 2 hours at 825 C- Zinc petalite..-.:;-'.. 8. 0

4 hours at 875 0..-.

5 {2 hours at 775 C- Crystal phases 2 hours at 825 O. }Zlnc petallte -z 4 hours at 875 C 2 hours at 775 10 2 hours at 825 0-... Z1110 petaltte 4 hours at 875 0.... u 2 hours at 775 C 4 hours at 850 0....}

4 hours at 850 0-.-.

2 hours at Zlnc petallte 13 hours at 8000 ""}Zlnc petallte..-

2 hours at 800 0.-.- Betaquartz 15 4 hours at 825: o..

2 how at 700 ----}Beta-quartz, cubic ZrO 16 6 hours at 860: 0.-

2 hours at 770 Beta-quartz, cubic 2:0,

17 6 hours at 870 0-.-. 2 hours at 800 0....

1g 2 ho s at 820 C Beta-quartz, zinc petallte...-

4 hours at 850 0....

2 hours at 800 0.-.. hours at }Zlnc petallte, beta quartz....

Loss

constant tangent LogR Tables I and II amply illustrate the composition and process parameters required for producing glass-ceramic articles according to this invention. The crystal content of the articles is greater than about 50% by weight and normally exceeds about 75% by weight, this depending upon the extent to which the components of the batch are adaptable to the formation of crystal phases. The crystals, themselves, are generally very uniformly fine-grained, substantially all being smaller than 5 microns in diameter and the vast majority being smaller than 1 micron diameter.

The preferred composition area comprises about 50- 70% SiO -25% A1 0 17-25% ZnO, 3-7% Zro0 with, optionally, up to 3% Ba() with heat treatments between about 800-9'25 C. being the most desirable to secure crystal assemblages consisting essentially of zinc petalite solid solution, beta-quartz solid solution, and tetragonal zirconia, only. Such products demonstrate a substantially zero expansion with excellent electrical properties.

Example I, when exposed to the heat treatment schedule reported in Table II, yields a very highly crystalline article wherein the crystals are uniformly extremely fine-grained (essentially all less than one micron in diameter), and which exhibits a high dielectric constant accompanied with a low loss tangent.

We claim:

1. A glass-ceramic article essentially free of alkali metal oxides and the alkaline earth metal oxides MgO, CaO, and SrO, exhibiting a coeflicient of thermal expansion 600 C.) of about -5 to +22 10""/ C., and wherein the crystal content thereof is greater than 50% by weight, said crystal content consisting essentially solely of crystals selected from the group consisting of zinc petalite solid solution and beta-quartz solid solution, the composition of said article being substantially the same throughout and consisting essentially, by weight on the oxide basis of about 13-40% ZnO, 9-26% A1 0 40-75% SiO and a nucleating agent selected from the group consisting of 3-10% ZrO- and 0.0010.5% of a noble metal selected from the group consisting of copper, gold, platinum, palladium, and silver.

2. A glass-ceramic article according to claim 1 wherein thereof is greater than 50% by weight, said crystal content consisting essentially solely of crystals selected from the group consisting of zinc petalite solid solution and beta-quartz solid solution, which comprises:

(a) melting a batch for a glass consisting essentially, by weight on the oxide basis, of about 15-45% ZnO, 10-28% A1 0 40-75% SiO and a nucleating agent selected from the group consisting of 3-10% Zr0 and 0.001-0.5%' of a noble metal selected from the group consisting of copper, gold, platinum, palladium, and silver;

(b) simultaneously cooling the melt below the transformation range thereof and shaping a glass body therefrom;

(c) subsequently exposing said glass body to a temperature between about 775 -950 C. for a period of time sufficient to crystallize said glass body in situ throughout; and then (d) cooling the crystallized body to room temperature.

4. A method according to claim 3 wherein said glass consists essentially, by weight on the oxide basis, of about 17-25% ZnO, 15-25% A1 0 50-70% SiO 3-7% Zr0 and 0-3% BaO.

5. A method according to claim 3 wherein said glass body is first exposed to a temperature between about 725 800 C. for about 2-6 hours, thereafter exposed to a temperature between about 800-925 C. for about l-8 hours, and then cooling to room temperature.

References Cited UNITED STATES PATENTS 3,352,698 11/1967 McMillan et a]. 106-39 DV 3,531,303 9/1970 Bahat 106-39 DV 3,252,811 5/ 1966 'Beall 106-39 DV 3,282,711 11/1966 Lin 106-39 DV 2,920,971 1/ 1960 Stookey 106-39 DV FOREIGN PATENTS 18,060 1962 Japan 106-39 DV 6516586 1967 Netherlands 106-39 DV OTHER REFERENCES Berezhnoi, A.I.; Glass-Ceramics and Photo-Sitalls; New York, 1970 (Moscow, 1966), pp. 216 and 218.

TOBIAS E. LEVOW, Primary Examiner W. R. SATIE'RFIELD, Assistant Examiner US. Cl. X.R.. 106-52; 65-33 

